Conventionally, the gain of such an amplifier is controlled via an electrical current. The current powers a "pump" optical source which supplies a "pump" wave for exciting the doping elements of the fiber. Known amplifiers include servo-control loops for regulating their gain. Such a loop measures the power of light radiated or amplified by the amplifier, said power being representative of the gain of the amplifier, and it being possible for said light to be referred to as "regulation light". The loop then compares the representative power with a reference power. Finally, it controls the powering current supplied to the pump source so as to limit or cancel the difference between the two powers.
In a first known amplifier, the regulation light is spontaneous emission light that is radiated transversely by the amplifying fiber. The first known amplifier is described in Document EP-A 517 503 (NTT).
In many industrial applications, that amplifier suffers from the drawbacks that the regulation light must be filtered, and that the power of the light as measured by a detector is both very low and also very sensitive to small variations in the situation of the detector relative to the fiber, and this disturbs regulation.
In a second known amplifier, the regulation light is spontaneous emission light which is guided and amplified by the amplifying fiber, and which is tapped at the output of that fiber. Therefore, the light has propagated along the fiber in the same direction as the signal to be amplified, and it may be called "codirectional".
The second known amplifier is described in Document EP-A-395 277 (STC-PLC). It suffers from the drawback that regulation light must be carefully filtered so that the measured power is not modified by the fact that the detector receives a portion of the light from the amplified signal. Such careful filtering increases the cost and the size of the amplifier.